Method for preparing a sorbent

ABSTRACT

A method is described for preparing a sorbent comprising the steps of: (i) forming agglomerates comprising a particulate support material, (ii) coating the agglomerates with a coating mixture powder comprising a particulate copper sulphide and a particulate calcined, rehydratable alumina to form a coated agglomerate, and (iii) drying the coated agglomerate to form a dried sorbent.

This invention relates to a method for preparing a sorbent, in particular a method for preparing sorbents comprising copper sulphide.

Copper sulphide containing sorbents may be used to remove heavy metals from fluid streams. Heavy metals such as mercury are found in small quantities in fluid streams such as hydrocarbon or other gas and liquid streams. Arsenic and antimony may also be found in small quantities in hydrocarbon streams. Mercury, in addition to its toxicity, can cause failure of aluminium heat exchangers and other processing equipment. Therefore there is a need to efficiently remove these metals from fluid streams, preferably as early as possible in the process flowsheet.

Copper sorbents are conventionally pelleted compositions or granules formed from precipitated compositions containing copper.

WO2009/101429 discloses a method for making an absorbent comprising the steps of: (i) forming a composition comprising a particulate copper compound capable of forming copper sulphide, a particulate support material, and one or more binders, (ii) shaping the composition to form an absorbent precursor, (iii) drying the absorbent precursor material, and (iv) sulphiding the precursor to form the absorbent. The sulphiding agent used to sulphide the absorbent precursor may be one or more sulphur compounds such as hydrogen sulphide, carbonyl sulphide, mercaptans and polysulphides, or mixtures of these. Hydrogen sulphide is preferred.

WO2011/021024 discloses a method for making a sorbent comprising the steps of: (i) applying, from a solution or a slurry, a layer of a copper compound on the surface of a support material, and (ii) drying the coated support material, wherein the thickness of the copper compound layer on the dried support is in the range 1-200 μm. In the Examples, the layer of copper compound was formed from a solution of copper amine carbonate or from a slurry of basic copper carbonate. The precursor was converted to a sorbent suitable for removing heavy metals from liquids or gases by applying one or more sulphur compounds to sulphide the copper compound and form CuS.

Whereas this method provides copper sorbents, there is a need to avoid having to employ a separate sulphiding step.

There is also a need to improve the product physical properties, notably the crush strength, which presently relies on the use of one or more binders.

Accordingly the invention provides a method for preparing a sorbent comprising the steps of:

-   -   (i) forming agglomerates comprising a particulate support         material,     -   (ii) coating the agglomerates with a coating mixture powder         comprising a particulate copper sulphide and a particulate         calcined, rehydratable alumina to form a coated agglomerate, and     -   (iii) drying the coated agglomerate to form a dried sorbent.

The invention further provides a sorbent obtainable by the method and the use of the sorbent in removing heavy metals, such as mercury, arsenic selenium, cadmium and antimony from heavy metal-containing fluid streams.

By “sorbent” we include absorbent and adsorbent.

By “calcined, rehydratable alumina” we mean a calcined amorphous or poorly crystalline transition alumina comprising one or more of rho-, chi- and pseudo gamma-aluminas. Such aluminas are capable of rehydration and can retain substantial amounts of water in a reactive form. Calcined, rehydratable aluminas are commercially available, for example as “CP alumina powders” available from BASF AG. They may be prepared, for example, by milling gibbsite (Al(OH)₃), to a 1-20 micron particle size followed by flash calcination for a short contact time as described in U.S. Pat. No. 2,915,365. In addition to gibbsite, amorphous aluminium hydroxide and other naturally found mineral crystalline hydroxides such as Bayerite and Nordstrandite or monoxide hydroxides, such as Boehmite (AIOOH) and Diaspore may be also used as a source of the calcined, rehydratable alumina.

The agglomerates may be described as inert “cores” on which the coating mixture powder comprising copper sulphide and calcined rehydratable alumina are coated. Because the cores are intended to be inert, copper compounds are not included. The agglomerates comprise a particulate support material and optionally one or more binders. Such support materials include alumina, metal-aluminate, silica, titania, zirconia, zinc oxide, aluminosilicates, zeolites, metal carbonate, silicon carbide, carbon, or a mixture thereof. The support material offers a means to adapt the physical properties of the sorbent to the duty. Thus the surface area, porosity and crush strength of the sorbent may suitably be tailored to its use. Support materials are desirably oxide materials, such as aluminas, titanias, zirconias, silicas and aluminosilicates, or mixtures of two or more of these. Hydrated oxides may also be used, for example alumina trihydrate or boehmite.

The particulate support material is desirably in the form of a powder, more preferably a powder with a D₅₀ particle size in the range 1-100 μm, preferably 1-20 μm, especially 1-10 μm.

A particularly suitable support material is a particulate calcined, rehydratable alumina. The particulate calcined, rehydratable alumina is preferably a calcined amorphous or poorly crystalline transition alumina comprising one or more of rho-alumina, chi-alumina and pseudo-gamma alumina. Preferably the particulate calcined, rehydratable alumina consists of one or more of rho-alumina, chi-alumina and pseudo-gamma alumina, especially rho-alumina. The particulate calcined, rehydratable alumina is desirably in the form of a powder, more preferably a powder with a D₅₀ particle size in the range 1-100 μm, preferably 1-20 μm, especially 1-10 μm. The BET Surface area of the calcined, rehydratable alumina as determined by nitrogen adsorption may be in the range 200-400 m²/g, preferably 250-300 m²/g.

Binders that may be used to prepare the agglomerates include clay binders such as bentonite, sepiolite, minugel and attapulgite clays; cement binders, particularly calcium aluminate cements such as ciment fondu; and organic polymer binders such as cellulose binders, or a mixture thereof. Particularly strong agglomerates may be formed where the binder is a combination of a cement binder and a clay binder. In such materials, the relative weights of the cement and clay binders may be in the range 1:1 to 3:1 (first to second binder). The total amount of the binder in the agglomerate may be in the range 5-30% by weight. The one or more binders are desirably in the form of powders, more preferably powder with a D₅₀ particle size in the range 1-100 μm, especially 1-20 μm.

However, we have found that where a particulate calcined, rehydratable alumina is used to prepare the agglomerates, they do not require the inclusion of a binder. Thus in a preferred embodiment, the agglomerates are formed simply by granulating, in the absence of other materials, a particulate calcined, rehydratable alumina consisting of an calcined amorphous alumina or a transition alumina comprising one or more of rho-alumina, chi-alumina and pseudo-gamma alumina, especially rho-alumina.

The support material is granulated in a granulator to form agglomerates, which provide a core essentially free of copper compounds. The agglomerates may be formed by mixing a powder composition with a little liquid, such as water, insufficient to form a slurry, and then causing the composition to agglomerate into roughly spherical granules in a granulator. The amount of liquid added will vary depending upon the porosity and wettability of the components, but may be 0.1 to 0.6 ml/g of support material. Aqueous or non-aqueous liquids may be used, but water is preferred. Granulator equipment is available commercially. The agglomerates preferably have a diameter in the range 1-15 mm.

The agglomerates may be aged and/or dried before coating to enhance their strength. Ageing and/or drying is preferably performed at 20-90° C. for 0.5-24 hours. Where calcined, rehydratable alumina is used as the support material the ageing temperature is preferably 40-90° C. An advantage of using just the calcined, rehydratable alumina to prepare the agglomerates is that the ageing step may be considerably reduced or eliminated. Thus ageing may be performed on alumina agglomerates prepared using a calcined, rehydratable alumina at 20-90° C., preferably 40-90° C. for a period of 0.5-8 hours, preferably 0.5-6 hours, more preferably 0.5-2 hours.

In a preferred embodiment, the agglomerates consist of the granulated particulate calcined, rehydratable alumina. Such agglomerates provide a high strength core on which to place the particulate copper sulphide coating.

The particulate copper sulphide used to prepare the sorbent may be sourced commercially or may be prepared by a number of methods. Suitable methods include roasting of copper or a copper compound with elemental sulphur, solvothermal processes, hydrothermal processes (e.g. microwave irradiation), electrodeposition techniques, precipitation of copper sulphide from solution, sulphiding of copper compounds using hydrogen sulphide, by electron irradiation, or by a mechanochemical process in which powdered copper metal is mixed with elemental sulphur under conditions that cause the elemental copper and elemental sulphur to react to form one or more copper sulphides. Such methods are described in the Materials Research Bulletin, vol 30, no 12, p 1495-1504, 1995.

Copper sulphides that may be used include Copper (II) sulphide, CuS, (covellite) and/or substoichiometric copper sulphides, e.g. of formula Cu_(2-x)S where x is 0-1, such as Cu₉S₅ (digenite). One or more copper sulphides may be used. Copper sulphides high in CuS are preferred, and the overall S:Cu atomic ratio of the particulate copper sulphide is preferably ≥0.8, more preferably ≥0.9, most preferably ≥0.95. Desirably, essentially all of the sulphided copper in the sorbent is in the form of copper (II) sulphide, CuS. The particulate copper sulphide may be in the form of a powder, preferably a preferably a powder with an average particle size, i.e. D₅₀, in the range 5-100 μm.

The copper sulphide content of the sorbent may be in the range 0.5-75% by weight (expressed as CuS in the dried material), but we have found that materials with low levels of copper sulphide are as effective in capturing heavy metals as conventional sorbent materials. Therefore the copper sulphide content of the sorbent is preferably 5-55% by weight, more preferably 5-45% by weight (expressed as CuS in the dried material).

The coating mixture comprises a particulate copper sulphide and a particulate calcined, rehydratable alumina. Preferably the particulate calcined, rehydratable alumina consists of a calcined amorphous alumina or a transition alumina comprising one or more of rho-alumina, chi-alumina and pseudo-gamma alumina, especially rho-alumina. The amount of copper sulphide in the coating mixture may be in the range 50-95% by weight, preferably 50-90% by weight, more preferably 55-75% by weight, most preferably 60-70% by weight.

The particulate calcined, rehydratable alumina in the coating mixture may be the same or different to a particulate calcined, rehydratable alumina used in the agglomerates but preferably is the same.

A binder may be included in the coating mixture but this is not necessary.

Other components are not necessary and so the coating mixture preferably consists of a particulate copper sulphide and a particulate calcined rehydratable alumina comprising a calcined amorphous alumina or a transition alumina comprising one or more of rho-alumina, chi-alumina and pseudo-gamma alumina, especially rho-alumina. Because of the calcined rehydratable alumina, binders are not required in the coating mixture.

The coating mixture may be prepared by simply mixing the particulate copper sulphide and the particulate calcined rehydratable alumina, using conventional blending techniques.

The coating mixture is combined with the agglomerates to form coated agglomerates that have a layer of particulate copper sulphide on their surface. This may be achieved conveniently by simply adding the coating mixture to the agglomerates as they are tumbled in the granulator. Other coating techniques may be used. The coated agglomerates may be formed with or without adding additional liquid. Minimizing the amount of liquid used advantageously reduces their drying time and reduces the possibility of forming agglomerates of the coating mixture itself which is undesirable. Furthermore, minimising water addition may reduce the formation of undesirable copper sulphates. Similarly, applying the coating mixture to the agglomerates under a non-oxidising atmosphere, such as oxygen-free nitrogen, may also help reduce the formation of sulphates. Additional liquid may however be required. The amount of liquid used may be 0.1 to 0.6 ml/g of coating mixture. Aqueous or non-aqueous liquids may be used, but water is preferred. The water may be mains water, demineralised water, deionised water and the like and is preferably low in dissolved mineral content. The liquid, if required, may conveniently be added by spraying.

The size of the coated agglomerates is largely determined by the size of the agglomerates. Thus the coated agglomerates preferably have a diameter in the range 1-15 mm.

The copper sulphide is present in a layer on the surface of the agglomerate. The thickness of the layer in the dried material may be in the range 1 to 2000 μm (micrometres), but preferably is in the range 1-1500 micrometres, more preferably 1-500 micrometres. Thinner layers make more efficient use of the applied copper.

A particularly preferred sorbent comprises a mixture of a particulate copper sulphide and a particulate calcined rehydratable alumina, coated as a surface layer of 1 to 2000 μm thickness on the surface of 1-15 mm agglomerates formed from a particulate calcined rehydratable alumina alumina support material. Accordingly the sorbent may be formed in the absence of a cement binder or a clay binder.

The coated agglomerates may be aged to enhance their strength before drying. Ageing of the calcined, rehydratable alumina-containing sorbents may be performed at 20-90° C., preferably 40-90° C. An advantage of using just the calcined, rehydratable alumina in the sorbent is that the ageing step may be considerably reduced or eliminated compared to prior art materials. Thus ageing may be performed on the coated agglomerates for 0.5-8 hours, preferably 0.5-6 hours, more preferably 0.5-2 hours before drying. Ageing under a non-oxidising atmosphere such as dry nitrogen reduces the potential for sulphate formation.

The coated agglomerates are dried. The drying temperature is preferably kept 200° C., more preferably 150° C. to avoid bulk decomposition of the copper sulphide. Drying temperatures up to 120° C. are more preferred, for example the coated agglomerate may conveniently be dried at about 70-120° C. Drying times may be in the range 0.25-16 hours. Drying under a non-oxidising atmosphere such as dry nitrogen may reduce sulphate formation.

The dried sorbent may be sieved to give a desired size fraction.

The sorbent may be used to treat both liquid and gaseous fluid streams containing heavy metals, in particular fluid streams containing mercury and/or arsenic. In one embodiment, the fluid stream is a hydrocarbon stream. The hydrocarbon stream may be a refinery hydrocarbon stream such as naphtha (e.g. containing hydrocarbons having 5 or more carbon atoms and a final atmospheric pressure boiling point of up to 204° C.), middle distillate or atmospheric gas oil (e.g. having an atmospheric pressure boiling point range of 177° C. to 343° C.), vacuum gas oil (e.g. atmospheric pressure boiling point range 343° C. to 566° C.), or residuum (atmospheric pressure boiling point above 566° C.), or a hydrocarbon stream produced from such a feedstock by e.g. catalytic reforming. Refinery hydrocarbon steams also include carrier streams such as “cycle oil” as used in FCC processes and hydrocarbons used in solvent extraction. The hydrocarbon stream may also be a crude oil stream, particularly when the crude oil is relatively light, or a synthetic crude stream as produced from tar oil or coal extraction for example. Gaseous hydrocarbons may be treated using the process, e.g. natural gas or refined paraffins or olefins, for example. Off-shore crude oil and off-shore natural gas streams in particular may be treated with the sorbent. Contaminated fuels such as petrol or diesel may also be treated. Alternatively, the hydrocarbon may be a condensate such as natural gas liquid (NGL) or liquefied petroleum gas (LPG), or gases such as a coal bed methane, landfill gas or biogas. Gaseous hydrocarbons, such as natural gas and associated gas are preferred.

Non-hydrocarbon fluid streams which may be treated using the sorbent include carbon dioxide, which may be used in enhanced oil recovery processes or in carbon capture and storage, solvents for decaffeination of coffee, flavour and fragrance extraction, solvent extraction of coal etc. Fluids, such as alcohols (including glycols) and ethers used in wash processes or drying processes (e.g. triethylene glycol, monoethylene glycol, Rectisol™, Purisol™ and methanol), may be treated by the inventive process. Mercury may also be removed from amine streams used in acid gas removal units. Natural oils and fats such as vegetable and fish oils may be treated by the process of the invention, optionally after further processing such as hydrogenation or transesterification e.g. to form biodiesel.

Other fluid streams that may be treated include the regeneration gases from dehydration units, such as molecular sieve off-gases, or gases from the regeneration of glycol driers.

The sorbent is of utility where the fluid stream contains water, preferably in low levels in the range 0.02 to 1% vol. Higher levels up to 5% vol may be tolerated for short periods. The sorbents may be regenerated simply after prolonged exposure to water simply by purging with a dry gas, preferably a dry inert gas such as nitrogen.

Preferably the absorption of heavy metal is conducted at a temperature below 150° C., preferably at or below 120° C. in that at such temperatures the overall capacity for heavy metal absorption is increased. Temperatures as low as 4° C. may be used. A preferred temperature range is 10 to 60° C. The gas hourly space velocity through the sorbent may be in the range normally employed.

Furthermore, the present invention may be used to treat both liquid and gaseous fluid streams containing one or more reductants such as hydrogen and/or carbon monoxide, notably hydrogen. In one embodiment, the fluid stream is a liquid hydrocarbon stream containing dissolved hydrogen and/or carbon monoxide. In another embodiment, the fluid stream is a gaseous stream containing hydrogen and/or carbon monoxide, i.e. a reducing gas stream. Gas streams that may benefit from this process include synthesis gas streams from conventional steam reforming processes and/or partial oxidation processes, and synthesis gas streams from a coal gasifier, e.g. as part of a IGCC process, after gas washing and heat recovery (cooling) steps, and before the sour shift stage. Other streams that may benefit from the present invention include refinery vent streams, refinery cracker streams, blast furnace gases, reducing gases, particularly hydrogen-rich gas streams, ethylene-rich streams and liquid or gaseous hydrocarbon streams, e.g. naphtha, fed or recovered from hydrotreating processes, such as hydrodesulphurisation or hydrodenitrification.

In use, the sorbent may be placed in a sorption vessel and the fluid stream containing heavy metal is passed through it. Desirably, the sorbent is placed in the vessel as one or more fixed beds according to known methods. More than one bed may be employed and the beds may be the same or different in composition.

The invention is further described by reference to the following Examples.

EXAMPLE 1

Agglomerates were formed by tumbling a calcined, rehydratable alumina powder in a rotating pan and adding water sprayed via a fine mist onto the alumina. The water content of the freshly granulated agglomerates before drying was found to be 29.7 wt %. Following granulation, the material was aged at 45° C. for 1 hour.

The properties of the calcined, rehydratable alumina powder were as follows:

(wt %) Chemical composition Residual Moisture (dried a 250° C. for 30 minutes) 2 Total loss on ignition (250-1100° C.) 7 SiO₂ <0.02 Fe₂O₃ <0.01 Na₂O <0.4 Physical Properties Surface area 270 m²/g Packed bulk density 38 lb/ft³ Particle size distribution (average size) 5 μm Particle size distribution (90 wt % <) 12 μm XRD Phase Amorphous

A coating mixture consisting of a copper sulphide powder and the same calcined, rehydratable alumina powder was then applied to the alumina agglomerates. The recipe for the coating mixture was as follows:

Component % Copper sulphide (99%) Eurolub 67 Calcined, rehydratable alumina (CP-5, BASF) 33

The amount of coating mixture was adjusted to produce a copper loading in the finished dried product of 18% wt (expressed as Cu). A coating layer (thickness about 1000 μm) was formed by adding the coating mixture to the alumina agglomerates in a rotating pan with further addition of water. Following coating, the material was aged at 45° C. before being dried in a fluid bed dryer at 105° C. The finished product was sieved to provide a sorbent particle size in the range 2.80-4.75 mm.

The physical properties were determined, and are shown below compared to a sulphided copper sorbent prepared using basic copper carbonate, cement and clay binders, and an alumina trihydrate (ATH) support material, according to the method described in WO2009/101429.

The tapped bulk density (TBD) was measured by pouring approximately 500 mls of sorbent granules into a 500 ml plastic measuring cylinder and tapping it until a constant volume was achieved. The TBD was calculated by dividing the mass of sorbent by the tapped volume.

The drum tumbling loss (DrTL) was measured by rotating 100 g of sorbent through 1800 total revolutions at 60 rpm for 30 minutes according to the ASTM method D4058-96. The DrTL is reported as a percentage of the original mass.

The mean crush strength (MCS) was determined by crushing 25 granules of each sorbent using an Engineering Systems C53 machine to calculate mean crush strength based on a normal distribution.

Ageing time TBD DrTL MCS Example Description (h) (g cm⁻³) (%) (kgF) 1(a) Agglomerate cores 1 0.86 0.00 3.82 1(b) Coated sorbent 1 0.91 0.00 5.46 material (5 minutes coating) 1(c) Coated sorbent 1 0.97 0.00 7.50 material (12 minutes coating) Comparative WO2009/101429 12 0.99 2.20 1.48

The use of a calcined, rehydratable alumina provided a much stronger product when compared to the prior art material produced using mixed binder and aluminium trihydrate. The rate at which strength develops also occurs much more rapidly in the calcined, rehydratable alumina product when compared to the mixed binder product with strength achieved 5 times higher following 1 h of ageing. 

1. A method for preparing a sorbent comprising the steps of: (i) forming agglomerates comprising a particulate support material in a granulator with a liquid, and ageing the agglomerates to form aged agglomerates, (ii) adding a coating mixture powder to the aged agglomerates in the granulator, wherein the coating mixture powder comprises a particulate copper sulphide and a particulate calcined, rehydratable alumina to form coated agglomerates, and (iii) drying the coated agglomerates to form a dried sorbent.
 2. A method according to claim 1, wherein the support material alumina, metal-aluminate, silicon carbide, silica, titania, zirconia, zinc oxide, an aluminosilicate, a zeolite, metal carbonate, carbon, or a mixture thereof.
 3. A method according to claim 1, wherein the support material is a particulate calcined, rehydratable alumina.
 4. A method according to claim 1, wherein the calcined rehydratable alumina comprises a calcined amorphous alumina or a transition alumina that is one or more of rho-alumina, chi-alumina, or pseudo gamma-alumina.
 5. A method according to claim 1, wherein the support material is in the form of a powder with a D₅₀ particle size in the range of 1-100 μm.
 6. A method according to claim 1, wherein a binder that is a clay binder, a cement binder, or an organic polymer binder is combined with the support material to form the agglomerates.
 7. A method according to claim 6 wherein the binder combined with the support material is a combination of a cement binder and a clay binder.
 8. A method according to claim 1, wherein agglomerates have a diameter in the range of 1-15 mm.
 9. A method according to claim 1, wherein the particulate copper sulphide material is manufactured by roasting copper or a copper compound with elemental sulphur, precipitating copper sulphide from solution, sulphiding copper compounds using hydrogen sulphide, or a mechanochemical process comprising mixing powdered copper metal with elemental sulphur under conditions that cause the elemental copper and elemental sulphur to react to form one or more copper sulphides.
 10. A method according to claim 1, wherein the copper sulphide comprises copper (II) sulphide and/or a substoichiometric copper sulphide of formula Cu_(2-x)S where x is 0-1.
 11. A method according to claim 1, wherein the particulate copper sulphide has a S:Cu atomic ratio of ≥0.8.
 12. A method according to claim 1, wherein the particulate copper sulphide is in the form of a powder with an average particle size, [D₅₀], in the range of 5-100 μm.
 13. A method according to claim 1, wherein the copper sulphide content of the sorbent is in the range of 0.5-75% by weight expressed as CuS in the dried sorbent.
 14. A method according to claim 1, wherein the copper sulphide content in the coating mixture is in the range of 50-95% by weight.
 15. A method according to claim 1, wherein the coating mixture consists of a particulate copper sulphide and particulate calcined, rehydratable alumina.
 16. A method according to claim 1, wherein the copper sulphide is present as a layer on the surface of the agglomerate and the thickness of the layer in the dried sorbent is in the range of 1 to 2000 micrometres.
 17. A method according to claim 1, wherein the sorbent comprises a mixture of a particulate copper sulphide and a particulate calcined, rehydratable alumina, coated as a surface layer of 1 to 2000 μm thickness on the surface of 1-15 mm agglomerates formed from a particulate calcined, rehydratable alumina support material.
 18. (canceled)
 19. A method according to claim 1, wherein the coating mixture is applied to the agglomerates under a non-oxidising atmosphere.
 20. A method according to claim 1, wherein the coated agglomerates are aged for 0.5-8 hours before drying.
 21. A method according to claim 1, wherein the sorbent is dried at a temperature up to 120° C. under a non-oxidising atmosphere.
 22. A sorbent obtained by the method of claim
 1. 23. A process for removing a heavy metal from a fluid stream comprising contacting the fluid stream with a sorbent prepared according to claim
 22. 